Emissions regulations for internal combustion engines have become more stringent over recent years. Environmental concerns have motivated the implementation of stricter emission requirements for internal combustion engines throughout much of the world. Governmental agencies, such as the Environmental Protection Agency (EPA) in the United States, carefully monitor the emission quality of engines and set acceptable emission standards, to which all engines must comply. Generally, emission requirements vary according to engine type. Emission tests for compression-ignition (diesel) engines typically monitor the release of diesel particulate matter (PM), nitrogen oxides (NOx), and unburned hydrocarbons (UHC). Catalytic converters implemented in an exhaust gas after-treatment system have been used to eliminate many of the pollutants present in exhaust gas. However, to remove diesel particulate matter, typically a diesel particulate filter (DPF) must be installed downstream from a catalytic converter, or in conjunction with a catalytic converter.
A common DPF comprises a porous ceramic matrix with parallel passageways through which exhaust gas passes. Particulate matter subsequently accumulates on the surface of the filter, creating a buildup which must eventually be removed to prevent obstruction of the exhaust gas flow. Common forms of particulate matter are ash and soot. Ash, typically a residue of burnt engine oil, is substantially incombustible and builds slowly within the filter. Soot, chiefly composed of carbon, results from incomplete combustion of fuel and generally comprises a large percentage of particulate matter buildup. Various conditions, including, but not limited to, engine operating conditions, mileage, driving style, terrain, etc., affect the rate at which particulate matter accumulates within a diesel particulate filter.
Accumulation of particulate matter typically causes backpressure within the exhaust system. Excessive backpressure on the engine can degrade engine performance. Particulate matter, in general, oxidizes in the presence of NO2 at modest temperatures, or in the presence of oxygen at higher temperatures. If too much particulate matter has accumulated when oxidation begins, the oxidation rate may get high enough to cause an uncontrolled temperature excursion. The resulting heat can destroy the filter and damage surrounding structures. Recovery can be an expensive process.
To prevent potentially hazardous situations, accumulated particulate matter is commonly oxidized and removed in a controlled regeneration process before excessive levels have accumulated. To oxidize the accumulated particulate matter, exhaust temperatures generally must exceed the temperatures typically reached at the filter inlet. Consequently, additional methods to initiate regeneration of a diesel particulate filter may be used. In one method, a reactant, such as diesel fuel, is introduced into an exhaust after-treatment system to initiate oxidation of particulate buildup and to increase the temperature of the filter. A filter regeneration event occurs when substantial amounts of soot are consumed on the particulate filter.
A controlled regeneration can be initiated by the engine's control system when a predetermined amount of particulate has accumulated on the filter, when a predetermined time of engine operation has passed, or when the vehicle has driven a predetermined number of miles. Oxidation from oxygen (O2) generally occurs on the filter at temperatures above about 400 degrees centigrade, while oxidation from nitric oxides (NO2), sometimes referred to herein as noxidation, generally occurs at temperatures between about 250 C and 400 C. Controlled regeneration typically consists of driving the filter temperature up to O2 oxidation temperature levels for a predetermined time period such that oxidation of soot accumulated on the filter takes place.
A controlled regeneration can become uncontrolled if the oxidation process drives the temperature of the filter upwards more than is anticipated or desired, sometimes to the point beyond which the filter substrate material can absorb the heat, resulting in melting or other damage to the filter. Less damaging uncontrolled or spontaneous regeneration of the filter can also take place at noxidation temperatures, i.e., when the filter temperature falls between about 250 C and 400 C. Such uncontrolled regeneration generally does not result in runaway temperatures, but can result in only partial regeneration of the soot on the filter. Partial regeneration can also occur when a controlled regeneration cannot continue because of a drop in temperature, exhaust gas flow rate, or the like. Partial regeneration and other factors can result in non-uniformity of soot distribution across the filter, resulting in soot load estimation inaccuracies and other problems.
The temperature of the particulate filter is dependent upon the temperature of the exhaust gas entering the particulate filter. Accordingly, the temperature of the exhaust must be carefully managed to ensure that a desired particulate filter inlet exhaust temperature is accurately and efficiently reached and maintained for a desired duration to achieve a controlled regeneration event that produces desired results.
Conventional systems use various strategies for managing the particulate filter inlet exhaust temperature. For example, some systems use a combination of air handling strategies, internal fuel dosing strategies, and external fuel dosing strategies.
The air handling strategies include managing an air intake throttle to regulate the air-to-fuel ratio. Lower air-to-fuel ratios, e.g., richer air/fuel mixtures, typically produce higher engine output exhaust temperatures.
Internal fuel dosing strategies include injecting additional fuel into the compression cylinders. Such in-cylinder injections include pre-injections or fuel injections occurring before a main fuel injection and post-injections or fuel injection occurring after a main fuel injection. Generally, post-injections include heat post-injections and non-heat post-injections. Heat post-injections are injections that participate along with the main fuel injection in the combustion event within the cylinder and occur relatively soon after the main fuel injection. Non-heat post injections are injections that occur later in the duty cycle compared to the heat post-injections and do not participate in the combustion event within the cylinder.
External fuel dosing strategies include injecting fuel into the exhaust gas stream at locations downstream of the engine. Typically, external fuel dosers are positioned in the exhaust aftertreatment system between the engine and a catalytic component, e.g., a diesel oxidation catalyst (DOC). The DOC reduces the number of pollutants in the exhaust gas through an oxidation process prior to the gas entering the particulate filter. The catalyst of the catalytic component must be at a specific temperature for oxidation of the pollutants to occur. The oxidation process heats the exhaust and causes the temperature of the exhaust to increase. In other words, during an oxidation process on the DOC, the DOC outlet exhaust temperature typically is greater than the DOC inlet exhaust temperature. Because fuel in the exhaust participates in the oxidation process, the exhaust temperature differential across the DOC, and thus the DOC outlet exhaust temperature, is largely dependent upon the amount of fuel in the exhaust gas entering the DOC.
Air handling strategies are aimed at controlling engine output exhaust temperatures. Internal fuel dosing strategies affect both engine output exhaust temperatures and DOC outlet exhaust temperatures. Fuel from internal fuel injections not combusted in the combustion event is oxidized in the DOC and increases the DOC outlet exhaust temperature. Similarly, external fuel injections simply add fuel to the exhaust stream, and thus increase the DOC outlet exhaust temperature.
In typical systems, the particulate filter 150 receives exhaust directly from the DOC. Accordingly, the particulate filter inlet exhaust temperature is approximately equal to the DOC outlet exhaust temperature. Therefore, an important tool in achieving a desired particulate filter inlet exhaust temperature is to ensure that the DOC is operating at the proper temperature for oxidation to occur. Because the temperature of the catalytic component is dependent upon the engine output exhaust temperature, many conventional engine systems employ various methods for controlling engine output exhaust temperature. However, such conventional methods can have several shortcomings. For example, for some mid-range engines operating under low load conditions and urban-type driving conditions (e.g., start-stop driving conditions with long periods of idle engine running), the engine output exhaust temperature control methods for such engines may be unable to achieve the engine output exhaust temperatures necessary to reach DOC activation temperatures. Moreover, when the engine is operating under low ambient air temperatures, it may be even more difficult for conventional engine output exhaust temperature control methods to attain adequately high engine output exhaust temperatures. When the DOC activation temperatures are not reached, the particulate filter may not adequately be able to regenerate, possibly resulting in the engine system outputting undesirable white smoke into the environment.
Based on the foregoing, a need exists for an engine controls strategy that achieves targeted engine output exhaust temperatures for achieving DOC activation temperatures and desired particulate filter inlet temperatures for regeneration events without the shortcomings of conventional engine output exhaust temperature control methods.